1. Field of the Invention
The present invention relates to a piezoelectric oscillator, a method of manufacturing such a piezoelectric oscillator, and an electronic device including such a piezoelectric oscillator and more particularly, the present invention relates to a piezoelectric oscillator, e.g., for use in a mobile communication device, a method of manufacturing such a piezoelectric oscillator, and an electronic device including such a piezoelectric oscillator.
2. Description of the Related Art
The oscillation frequency of each piezoelectric oscillator is determined by the resonance frequency of a piezoelectric resonator and a combined capacitance, which is equal to the sum of capacitance components connected to the piezoelectric resonator. The resonance frequency and the combined capacitance are designed so as to output a predetermined oscillation frequency. However, a problem arises in that when heat is applied to the piezoelectric resonator during a soldering process or other process during assemblage, thermal hysteresis causes the resonance frequency of the piezoelectric resonator to change, so that the oscillation frequency of the piezoelectric oscillator deviates from the designed value.
Accordingly, in conventional piezoelectric oscillators, a trimmer capacitor connected to a piezoelectric resonator is provided to correct the deviation from the oscillation frequency that is caused by the thermal hysteresis. In particular, the trimmer capacitor is adjusted so that the frequency corresponding to the deviation from the oscillation frequency can be corrected. Thus, the combined capacitance of the piezoelectric oscillator is changed so that the oscillation frequency is equal to the designed value. Examples of the trimmer capacitor include a rotational drive type trimmer capacitor including a rotor electrode, a stator electrode, and a dielectric sandwiched between the rotor and stator electrodes, a laser trimmer capacitor in which a part of an electrode disposed on a substrate is cut two-dimensionally by a laser (a part of the plane is removed). Generally, the size of the rotational drive type trimmer capacitor is about 3 mm in widthxc3x973 mm in length. The size of the laser trimmer capacitor is about 3 mm in widthxc3x975 mm in length.
The conventional piezoelectric oscillator is disclosed, e.g., in Japanese Unexamined Patent Application Publication No. 7-99411.
The capacitance of the rotational drive type trimmer capacitor is determined by setting the position of the rotor electrode relatively to the stator electrode. Thus, a problem arises in that the capacitance is changed by impact, which may be caused by falling and other external forces. Moreover, the rotational drive type trimmer capacitor has a rotation mechanism to determine the position of the stator electrode relative to the rotor electrode. Thus, a problem arises in that the area occupied by the circuit substrate is large, and reduction in size of the piezoelectric oscillator containing the rotational drive type trimmer capacitor is impossible. Also, problematically, the rotational drive type trimmer capacitor is very expensive. Reduction in cost of the piezoelectric oscillator using the rotational drive type trimmer capacitor can not be achieved.
The capacitance of the laser trimmer capacitor can be easily determined by burning-cutting the capacitor in a predetermined position thereof. Thus, no problem arises with the capacitance deviating due to impact caused by falling or other applied force. However, the area occupied by the circuit substrate is large, since the circuit substrate is required to have a plane for use in burning-cutting by the laser. Thus, the size of the piezoelectric oscillator using the laser trimmer capacitor can not be reduced.
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric oscillator having a greatly reduced size and cost, and an electronic device including such a piezoelectric oscillator and having a greatly reduced size and cost.
Also, other preferred embodiments of the present invention provide a method of producing a piezoelectric oscillator by which the piezoelectric oscillator can be efficiently produced.
According to a preferred embodiment of the present invention, a piezoelectric oscillator includes a piezoelectric resonator, an oscillation circuit connected to the piezoelectric resonator, a first capacitance element having a first terminal that is grounded and a second terminal that is connected to at least one terminal of the piezoelectric resonator, and a series circuit including a resistor of which the resistance can be adjusted and a second capacitance element connected in parallel to the first capacitance element.
Preferably, one terminal of the resistor is grounded, and the other terminal is connected to the second capacitance element.
Preferably, the piezoelectric oscillator in accordance with a preferred embodiment of the present invention further includes a temperature compensation circuit connected to the piezoelectric oscillator resonator.
Preferably, the piezoelectric oscillator in accordance with a preferred embodiment of the present invention further includes a variable-capacitance diode having one terminal that is connected to the piezoelectric resonator.
Also, preferably, the first capacitance element includes the variable capacitance diode.
Preferably, the resistor is capable of being trimmed by a laser or other suitable process.
Also, preferably, the resistor is a thick-film resistor.
Also, according to a preferred embodiment of the present invention, a method of producing a piezoelectric oscillator including a resistor for adjustment of the frequency thereof, includes the steps of printing the circuit elements of a resonance circuit including the resistor on a circuit substrate, mounting a circuit element on the circuit substrate, and trimming the resistor using a laser.
Also, according to a preferred embodiment of the present invention, a method of producing a piezoelectric oscillator including a resistor for adjustment of the frequency thereof, including the steps of printing circuit elements on a circuit substrate, mounting circuit elements for a resonance circuit including the resistor on the circuit substrate, and trimming the resistor using a laser.
According to another preferred embodiment of the present invention, an electronic device includes at least one piezoelectric oscillator according to the preferred embodiments described above.
Also, according to another preferred embodiment of the present invention, an electronic device which includes at least one piezoelectric oscillator produced by the above-described method of producing a piezoelectric oscillator.
The combined capacitance of the piezoelectric oscillator in accordance with preferred embodiments of the present invention can be changed by changing the resistance, so that the oscillation frequency can be adjusted. Accordingly, the piezoelectric oscillator achieves significant reductions in size and cost, since a trimmer capacitor does not need to be used.
According to the method of producing a piezoelectric oscillator of preferred embodiments of the present invention, a process for adjusting the resistance, is preferably performed by trimming using a laser. Thus, efficient production of the piezoelectric oscillator is achieved.
An electronic device in accordance with a preferred embodiment of the present invention includes the piezoelectric oscillator in accordance with other preferred embodiments of the present invention, having greatly reduced size and cost. Thus, the size and cost of the electronic device is greatly reduced.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.